This invention relates to an automatic package strapping machine for tightening a thermoplastic strap looped around a package. More particularly, it relates to an automatic package strapping machine capable of tightening a thermoplastic strap looped around a package even with well-adjusted, gentle force in order that the contents of the package may not be damaged by tightening of the strap.
Package strapping machines adapted to automatically strap a package with a thermoplastic strap drawn off a reel and weld overlapping ends of the strap by heat and pressure are widely used, because they are efficient and save manpower.
The structure of general automatic package strapping machines will now be described with reference to FIG. 8. A sliding plate 82 is horizontally supported to slide back and forth in a horizontal direction by a table 81 on which a package A to be strapped is loaded. Disposed just under the sliding plate 82 are a first clamp 83, a pressing device 84, and a second clamp 85, mounted so that they can each move up and down freely by means of a group of cams 87 secured to a cam shaft 86 driven by a motor M.sub.1. Provided between the first clamp 83 and the upper end of the pressing device 84 is a cutting blade 88 adapted to cut a thermoplastic strap B. Provided between the sliding plate 82 and the pressing device 84 is a heater (not shown) that can move back and forth therebetween.
Thermoplastic strap B is drawn off a reel (not shown) and allowed to pass through a pair of upper and lower rolls 90, 89 which are rotated by motor M.sub.2 to feed forward and then tighten the strap B looped around the package. The strap is fed through a strap guide 91, and a guiding groove 92 of the first clamp 83, where it is introduced into an arch (not shown) standing upright on the table 81, and then looped around the package A. A leading end of the strap is then fed beneath the sliding plate 82. The motor M.sub.2 and thus the forward feeding of the strap B stops when the leading end of the strap B presses a switch (not shown).
Upper roll 90 is forwardly and backwardly rotated by the motor M.sub.2, while lower roll 89 is pressed firmly against the upper roll 90 by a spring 93, so that their nipping force allows the strap to move. The lower roll 89 is also designed to increase the nipping force in association with the upper roll 90 when the strap is required to be tightened by means of a roller 95 and a spring 96 actuated by a cam 94 secured to the cam shaft 86. Because of the nipping force increase, the strap looped around the package A can be fastened without causing slip between the rolls 89, 90 and the strap.
While the first clamp 83 holds the leading end of the strap B, backward rotation of the roll 90 by the motor M.sub.2 tightens the strap. Overlapping portions of the strap are then held against the sliding plate 82 by the second clamp 85, and the heater is inserted between the overlapping portions in the area between the two clamps. The pressing device moves up to first cut the trailing end of the strap with the cutting blade 88, and then press the overlapping portions and the heater together against the sliding plate 82. The heater subsequently withdraws from between the overlapping portions while the pressing device continues to press on the overlapping portions, thus welding them together. When the welding is complete the clamps 83 and 85 and the pressing device 84 are lowered.
When the welding of the thermoplastic strap is over, the sliding plate moves backward from its forward position, in order to withdraw itself from between the welded portion of the strap and the package. At the withdrawal of the sliding plate, the welded portion of the strap, which had been separated from an under surface of the package by the sliding plate, comes into contact with the package by its own tension, because of strong tightening of the strap. The sliding plate then returns to the initial forward position, passing by just under the welded portion of the strap, completing one cycle of the package-strapping machine.
However, when the thermoplastic strap is gently tightened to take into account the variety or the material of the package contents, there will form a slack in the strap under the package and sliding plate on the conventional package strapping machine. Because the welded portion of the strap remains slackened under the package, the sliding plate, which is withdrawn from between the strap and the package, can reenter the strap, or butt against the strap when returned to the forward position. This can make it difficult to remove the package from the machine, or can impede the next strap-forwarding and tightening cycle.